Various information recording techniques have been developed along with recent increase of capacity in information processing. Particularly, the areal recording density of HDDs using a magnetic recording technique has continued to increase at an annual rate of about 100%. Recently, a 2.5-inch perpendicular magnetic recording medium used for an HDD or the like has been required to have an information storage capacity greater than 160 GB per disk. In order to meet such a demand, it is necessary to achieve an information recording density greater than 250 Gbit/inch2.
In recent years, perpendicular magnetic recording media of perpendicular magnetic recording type have been proposed in order to achieve a higher recording density of a magnetic recording medium used for an HDD or the like. In a perpendicular magnetic recording type, an axis of easy magnetization in a magnetic recording layer is arranged so as to be oriented in a direction perpendicular to a surface of a substrate. The perpendicular magnetic recording type is suitable to increase the recording density because it can suppress heat fluctuation phenomena as compared to a conventional longitudinal magnetic recording type.
As a magnetic recording medium of perpendicular magnetic recording type, a CoCrPt—SiO2 perpendicular magnetic recording medium has been proposed because it exhibits high thermal stability and excellent recording characteristics (see Non-patent Document 1). A granular structure in which non-magnetic grain boundary portions are formed between magnetic grains that have grown continuously in a columnar shape is formed in a magnetic recording layer, thereby reducing the size of the magnetic grains and improving a magnetic coercive force Hc. It has been known that an oxide is used for non-magnetic grain boundaries (non-magnetic portions between magnetic grains). For example, there has been proposed to use one of SiO2, Cr2O3, TiO, TiO2, and Ta2O5 (Patent Document 1).
Furthermore, a size-reduction promoting layer having a non-magnetic granular structure (also referred to as an onset layer in some cases) in which SiO2 is segregated at grain boundaries of a non-magnetic metal of CoCr may be provided below a magnetic recording layer (Patent Document 2). The size-reduction promoting layer is deposited on a Ru underlayer, which is a continuous crystal that is not isolated. Therefore, when a granular layer is to be formed on a Ru layer, segregation is not necessarily completed at an initial stage. The crystal orientation of bases of the granular columns is disordered, and the bases of the granular columns spread laterally such that they are coupled to each other. The disorder of the crystal orientation causes decrease of the SNR or the magnetic coercive force Hc. The coupling of the crystal grains also causes decrease of the SNR. In this regard, the size-reduction promoting layer allows the bases of the granular portions to be formed of non-magnetic substance and thus serves to promote segregation and isolation of the granular portions (magnetic grains) in the magnetic recording layer even if the bases of granular portions have been coupled to each other.    Non-patent Document 1: T. Oikawa et al., IEEE Trans. Magn, vol. 38, 1976-1978 (2002)    Patent Document 1: JP-A-2006-024346    Patent Document 2: JP-A-2006-268972